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authorBjörn Gustavsson <[email protected]>2019-01-19 06:26:50 +0100
committerBjörn Gustavsson <[email protected]>2019-01-24 10:15:32 +0100
commit17f9cfaf93f7b46087eff39c4cf4da7b07df5b11 (patch)
tree3f671b8d5e33a63372d48d1c2a740a205475c2d4 /lib
parenta0104bc16c8c6f57c2725d07b811bf3bcb0a2455 (diff)
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Introduce optimizations of tail phis
Try to eliminate short blocks that starts with a phi node and end in a return. For example: Result = phi { Res1, 4 }, { literal true, 5 } Ret = put_tuple literal ok, Result ret Ret The code in this block can be inserted at the end blocks 4 and 5. Thus, the following code can be inserted into block 4: Ret:1 = put_tuple literal ok, Res1 ret Ret:1 And the following code into block 5: Ret:2 = put_tuple literal ok, literal true ret Ret:2 Which can be further simplified to: ret literal {ok, true} This transformation may lead to more code improvements, for example: * Stack trimming * Fewer test_heap instructions * Smaller stack frames
Diffstat (limited to 'lib')
-rw-r--r--lib/compiler/src/beam_ssa_opt.erl164
1 files changed, 163 insertions, 1 deletions
diff --git a/lib/compiler/src/beam_ssa_opt.erl b/lib/compiler/src/beam_ssa_opt.erl
index 3685c09a2b..2c898ba6f8 100644
--- a/lib/compiler/src/beam_ssa_opt.erl
+++ b/lib/compiler/src/beam_ssa_opt.erl
@@ -39,7 +39,7 @@
-include("beam_ssa_opt.hrl").
--import(lists, [append/1,duplicate/2,foldl/3,keyfind/3,member/2,
+-import(lists, [all/2,append/1,duplicate/2,foldl/3,keyfind/3,member/2,
reverse/1,reverse/2,
splitwith/2,sort/1,takewhile/2,unzip/1]).
@@ -142,6 +142,7 @@ finish_1([], _StMap) ->
prologue_passes(Opts) ->
Ps = [?PASS(ssa_opt_split_blocks),
?PASS(ssa_opt_coalesce_phis),
+ ?PASS(ssa_opt_tail_phis),
?PASS(ssa_opt_element),
?PASS(ssa_opt_linearize),
?PASS(ssa_opt_tuple_size),
@@ -157,6 +158,7 @@ repeated_passes(Opts) ->
?PASS(ssa_opt_bs_puts),
?PASS(ssa_opt_dead),
?PASS(ssa_opt_cse),
+ ?PASS(ssa_opt_tail_phis),
?PASS(ssa_opt_type_continue)], %Must run after ssa_opt_dead to
%clean up phi nodes.
passes_1(Ps, Opts).
@@ -431,6 +433,160 @@ c_fix_branches([{_,Pred}|As], L, Blocks0) ->
c_fix_branches([], _, Blocks) -> Blocks.
%%%
+%%% Eliminate phi nodes in the tail of a function.
+%%%
+%%% Try to eliminate short blocks that starts with a phi node
+%%% and end in a return. For example:
+%%%
+%%% Result = phi { Res1, 4 }, { literal true, 5 }
+%%% Ret = put_tuple literal ok, Result
+%%% ret Ret
+%%%
+%%% The code in this block can be inserted at the end blocks 4 and
+%%% 5. Thus, the following code can be inserted into block 4:
+%%%
+%%% Ret:1 = put_tuple literal ok, Res1
+%%% ret Ret:1
+%%%
+%%% And the following code into block 5:
+%%%
+%%% Ret:2 = put_tuple literal ok, literal true
+%%% ret Ret:2
+%%%
+%%% Which can be further simplified to:
+%%%
+%%% ret literal {ok, true}
+%%%
+%%% This transformation may lead to more code improvements:
+%%%
+%%% - Stack trimming
+%%% - Fewer test_heap instructions
+%%% - Smaller stack frames
+%%%
+
+ssa_opt_tail_phis({#st{ssa=SSA0,cnt=Count0}=St, FuncDb}) ->
+ {SSA,Count} = opt_tail_phis(SSA0, Count0),
+ {St#st{ssa=SSA,cnt=Count}, FuncDb}.
+
+opt_tail_phis(Blocks, Count) when is_map(Blocks) ->
+ opt_tail_phis(maps:values(Blocks), Blocks, Count);
+opt_tail_phis(Linear0, Count0) when is_list(Linear0) ->
+ Blocks0 = maps:from_list(Linear0),
+ {Blocks,Count} = opt_tail_phis(Blocks0, Count0),
+ {beam_ssa:linearize(Blocks),Count}.
+
+opt_tail_phis([#b_blk{is=Is0,last=Last}|Bs], Blocks0, Count0) ->
+ case {Is0,Last} of
+ {[#b_set{op=phi,args=[_,_|_]}|_],#b_ret{arg=#b_var{}}=Ret} ->
+ {Phis,Is} = splitwith(fun(#b_set{op=Op}) -> Op =:= phi end, Is0),
+ case suitable_tail_ops(Is) of
+ true ->
+ {Blocks,Count} = opt_tail_phi(Phis, Is, Ret,
+ Blocks0, Count0),
+ opt_tail_phis(Bs, Blocks, Count);
+ false ->
+ opt_tail_phis(Bs, Blocks0, Count0)
+ end;
+ {_,_} ->
+ opt_tail_phis(Bs, Blocks0, Count0)
+ end;
+opt_tail_phis([], Blocks, Count) ->
+ {Blocks,Count}.
+
+opt_tail_phi(Phis0, Is, Ret, Blocks0, Count0) ->
+ Phis = rel2fam(reduce_phis(Phis0)),
+ {Blocks,Count,Cost} =
+ foldl(fun(PhiArg, Acc) ->
+ opt_tail_phi_arg(PhiArg, Is, Ret, Acc)
+ end, {Blocks0,Count0,0}, Phis),
+ MaxCost = length(Phis) * 3 + 2,
+ if
+ Cost =< MaxCost ->
+ %% The transformation would cause at most a slight
+ %% increase in code size if no more optimizations
+ %% can be applied.
+ {Blocks,Count};
+ true ->
+ %% The code size would be increased too much.
+ {Blocks0,Count0}
+ end.
+
+reduce_phis([#b_set{dst=PhiDst,args=PhiArgs}|Is]) ->
+ [{L,{PhiDst,Val}} || {Val,L} <- PhiArgs] ++ reduce_phis(Is);
+reduce_phis([]) -> [].
+
+opt_tail_phi_arg({PredL,Sub0}, Is0, Ret0, {Blocks0,Count0,Cost0}) ->
+ Blk0 = map_get(PredL, Blocks0),
+ #b_blk{is=IsPrefix,last=#b_br{succ=Next,fail=Next}} = Blk0,
+ case is_exit_bif(IsPrefix) of
+ false ->
+ Sub1 = maps:from_list(Sub0),
+ {Is1,Count,Sub} = new_names(Is0, Sub1, Count0, []),
+ Is2 = [sub(I, Sub) || I <- Is1],
+ Cost = build_cost(Is2, Cost0),
+ Is = IsPrefix ++ Is2,
+ Ret = sub(Ret0, Sub),
+ Blk = Blk0#b_blk{is=Is,last=Ret},
+ Blocks = Blocks0#{PredL:=Blk},
+ {Blocks,Count,Cost};
+ true ->
+ %% The block ends in a call to a function that
+ %% will cause an exception.
+ {Blocks0,Count0,Cost0+3}
+ end.
+
+is_exit_bif([#b_set{op=call,
+ args=[#b_remote{mod=#b_literal{val=Mod},
+ name=#b_literal{val=Name}}|Args]}]) ->
+ erl_bifs:is_exit_bif(Mod, Name, length(Args));
+is_exit_bif(_) -> false.
+
+new_names([#b_set{dst=Dst}=I|Is], Sub0, Count0, Acc) ->
+ {NewDst,Count} = new_var(Dst, Count0),
+ Sub = Sub0#{Dst=>NewDst},
+ new_names(Is, Sub, Count, [I#b_set{dst=NewDst}|Acc]);
+new_names([], Sub, Count, Acc) ->
+ {reverse(Acc),Count,Sub}.
+
+suitable_tail_ops(Is) ->
+ all(fun(#b_set{op=Op}) ->
+ is_suitable_tail_op(Op)
+ end, Is).
+
+is_suitable_tail_op({bif,_}) -> true;
+is_suitable_tail_op(put_list) -> true;
+is_suitable_tail_op(put_tuple) -> true;
+is_suitable_tail_op(_) -> false.
+
+build_cost([#b_set{op=put_list,args=Args}|Is], Cost) ->
+ case are_all_literals(Args) of
+ true ->
+ build_cost(Is, Cost);
+ false ->
+ build_cost(Is, Cost + 1)
+ end;
+build_cost([#b_set{op=put_tuple,args=Args}|Is], Cost) ->
+ case are_all_literals(Args) of
+ true ->
+ build_cost(Is, Cost);
+ false ->
+ build_cost(Is, Cost + length(Args) + 1)
+ end;
+build_cost([#b_set{op={bif,_},args=Args}|Is], Cost) ->
+ case are_all_literals(Args) of
+ true ->
+ build_cost(Is, Cost);
+ false ->
+ build_cost(Is, Cost + 1)
+ end;
+build_cost([], Cost) -> Cost.
+
+are_all_literals(Args) ->
+ all(fun(#b_literal{}) -> true;
+ (_) -> false
+ end, Args).
+
+%%%
%%% Order element/2 calls.
%%%
%%% Order an unbroken chain of element/2 calls for the same tuple
@@ -2116,3 +2272,9 @@ sub_arg(Old, Sub) ->
#{Old:=New} -> New;
#{} -> Old
end.
+
+new_var(#b_var{name={Base,N}}, Count) ->
+ true = is_integer(N), %Assertion.
+ {#b_var{name={Base,Count}},Count+1};
+new_var(#b_var{name=Base}, Count) ->
+ {#b_var{name={Base,Count}},Count+1}.